While silicon wafers or sheets may be used in the integrated circuit industry, these silicon wafers or sheets also may be used in the solar cell industry. The majority of solar cells are made from silicon wafers, such as single crystal silicon wafers. Currently, a major cost of a crystalline silicon solar cell is the wafer on which the solar cell is made. The efficiency of the solar cell, or the amount of power produced under standard illumination, is limited, in part, by the quality of this wafer. As the demand for solar cells increases with demand for green energy, one goal of the solar cell industry is to lower the cost/power ratio. Any reduction in the cost of manufacturing a wafer without decreasing quality will lower this cost/power ratio and enable the wider availability of this clean energy technology.
The highest efficiency silicon solar cells may have an efficiency of greater than 20%. These are made using electronics-grade monocrystalline silicon wafers. Such wafers may be made by sawing thin slices from a monocrystalline silicon cylindrical boule grown using the Czochralski method. These slices may be less than 200 μm thick. The subsequent sawing process leads to approximately 200 μm of kerf loss, or loss due to the width of a saw blade, per wafer. The cylindrical boule or wafer also may need to be squared off to make a square solar cell. Both the squaring and kerf losses lead to material waste and increased material costs. As solar cells become thinner, the percent of silicon waste per cut increases. Limits to sawing technology may hinder the ability to obtain thinner solar cells.
Other solar cells are made using wafers sawed from polycrystalline silicon ingots. Polycrystalline silicon ingots may be grown faster than monocrystalline silicon. However, the quality of the resulting wafers is lower because there are more defects or grain boundaries and this lower quality results in lower efficiency solar cells. The sawing process for a polycrystalline silicon ingot; is as inefficient as a monocrystalline silicon ingot or boule.
Another solution that may reduce silicon waste is cleaving a wafer from a silicon ingot after ion implantation. For example, hydrogen, helium, or other noble gas ions are implanted beneath the surface of the silicon ingot to form an implanted region. This is followed by a thermal, physical, or chemical treatment to cleave the wafer from the ingot along this implanted region. While cleaving through ion implantation can produce wafers without kerf losses, it has yet to be proven that this method can be employed to produce silicon wafers economically.
Yet another solution is to pull a ribbon of silicon vertically from a melt and then allow the pulled silicon to cool and solidify into a sheet. The removed latent heat during the cooling and solidifying must be removed along the vertical ribbon. This results in a large temperature gradient along the ribbon. This temperature gradient stresses the crystalline silicon ribbon and may result in poor quality multi-grain silicon. The width and thickness of the ribbon also may be limited due to this temperature gradient.
Producing sheets horizontally from a melt may be less expensive than silicon sliced from an ingot and may eliminate kerf loss or loss due to squaring. Sheets produced horizontally from a melt also may be less expensive than silicon cleaved from an ingot using hydrogen ions or other vertically-pulled silicon ribbon methods. Furthermore, separating a sheet horizontally from a melt may improve the crystal quality of the sheet compared to vertically-pulled ribbons. A crystal growth method such as this that can reduce material costs would be a major enabling step to reduce the cost of silicon solar cells. However, the thickness of this sheet may need to be uniform or a specific value for certain solar cell designs. Thus, the thickness may need to be controlled during production. Many thickness measurement devices cannot withstand the high temperature environment of the melt. There is a need in the art for measuring a sheet in a melt and, more particularly, measuring the thickness of a sheet in a melt.